So for the car:
<-- This is constant.
The position of the car is given as
For the cop:
The position of the cop is given as
We want to find where the car and cop meet, so equate these two expressions:
Now solve for t.
I get (which we already knew) and
It is possible that the computer didn't accept your second (and correct) answer because you didn't give the result to the proper number of significant digits: t = 22.7 s.